Lipids constituting the biomembranes of plants changes from the liquid crystal form into the solid form depending on the lowering of surrounding temperature, and the properties of the biomembranes are also changed therewith. It is believed in the solid state that the membrane losses the selectivity of material permeability, become incapable of effecting the essential functions, and thus the cells are impaired. Among the lipids, phosphatidyl glycerol, referred to hereinafter as PG, is a lipid easily solidified at a high temperature which has a high transition temperature from the liquid crystal to the solid state. Thus, the sensitivity of the biomembrane to temperature varies depending on the properties of PG. In this connection, the easy solidification property of PG is determined by the kinds of fatty acids as the constituents of it. The transfer of the fatty acid to glycerol-3-phosphate, referred to hereinafter as G-3-P, is carried out by G-3-P acyltransferase, referred to hereinafter as ATase, of chlorophyll. In other words, the transfer reaction of the fatty acid portion from the complex of the fatty acid and an acyl carrier protein, referred to hereinafter as ACP, to the G-3-P is catalyzed by the ATase.
In plants, the synthesis of fatty acids is carried out solely in chlorophyll, and the complex of the fatty acid and the ACP as the substrate of the ATase comprises primarily palmitoyl-ACP, referred to hereinafter as 16:0-ACP, and oleoyl-ACP, referred to hereinafter as 18:1-ACP. The selection of the substrates by the ATase is determined by the properties of the ATase itself, that is the substrate selectivity of the ATase. The substrate selectivities of the ATase have been examined in a variety of plants. For example, the ATases of spinach and pea as chilling resistant plants have high substrate specificity to 18:1-ACP, and the PG of these plants are in the liquid crystal state even at a relatively low temperature (Eur. J. Biochem. 129 (1983) 629-636). By contrast, the ATase of a chilling sensitive plant such as squash cannot distinguish 16:0-ACP and 18:1-ACP and transfer the fatty acids in respective complexes at the substantially equal ratio, so that the PG of the squash solidifies at a relatively high temperature (as described in detail below). Further, on measuring the substrate selectivities, the selectivities of fatty acid thioesters can be examined with either case of using ACP and CoA (Coenzyme A) (Plant Physiol. 83 (1987) 676-680).
Among the ATases of the chilling resistant plants, only the ones of Arabidopsis thaliana (Japanese Patent Laid-Open Publication No. 11891/1992; Japanese Patent Application No. 4782/1990), pea (Plant Mol. Biol. 17 (1991) 1067-1076) and spinach disclosed by the present inventor (WO 95/14094, International Application PCT/JP94/01956) have the overall amino acid sequences which have been completely elucidated. It has been revealed that the integration of an ATase gene derived from Arabidopsis thaliana as a chilling resistant plant or from squash as a chilling sensitive plant into tobacco as a plant having medium temperature sensitivity by the technology of genetic engineering permits the temperature sensitivity of tobacco to change into further chilling resistant in the case of the Arabidopsis thaliana and into further chilling sensitive in the case of the squash (Japanese Patent Publication No. 504439/1994; Japanese Patent Application No. 502792/1992).